This application is related to U.S. Patent Application Ser. No. 10/024,416 entitled, xe2x80x9cMethod for Measuring the Response of a Voltage Controlled Oscillatorxe2x80x9d(Common Assignee) filed concurrently herewith.
This invention relates to adjusting the frequency of a voltage-controlled oscillator in order to match the timing of pixels to the actual time of flight on a raster output scanner. The described here eliminates a customary inductor, allowing the elimination of a costly manufacturing adjustment.
Referring to FIG. 1, there is shown a tangential (fast-scan) view of a prior art raster output scanner 10 of a printing system. The raster scanning system 10 utilizes a laser light source 12, a collimator 14, pre-polygon optics 16, a multi-faceted rotating polygon mirror 18 as the scanning element, post polygon optics 20 and a photosensitive medium 22.
The laser light source 12 sends a light beam 24 to the rotating polygon mirror 18 through the collimator 14 and the pre-polygon optics 16. The collimator 14 collimates the light beam 24 and the pre-polygon optics 16 focuses the light beam 24 in the sagittal or cross-scan plane onto the rotating polygon mirror 18. The facets 26 of the rotating polygon mirror 18 reflect the light beam 24 and cause the reflected light beam 24 to revolve about an axis near the reflection point of the facet 26. The reflected light beam 24 is utilized through the post polygon optics 20 to scan a document at the input of an imaging system or can be used to impinge upon a photographic film or a photosensitive medium 22, such as a xerographic drum at the output of an imaging system. Hereinafter, for the purpose of simplicity the xe2x80x9crotating polygon mirrorxe2x80x9d will be referred to as xe2x80x9cpolygonxe2x80x9d.
In this process, depending on the manufacturing tolerances, each facet might have different characteristics such as a minute width variation which can cause the line scanned by this facet to be scanned faster or slower than average scan time. This type of error is called facet to facet error. In order to correct this problem, it is best to check the error of each facet compared to the average speed of the polygon (average scan time) which is the average speed of all the facets of the polygon.
To find the facet to facet or the reference frequency errors, the time difference between the arrival times of the end of scan and end of count signals of each facet has to be measured. Typically, an analog phase detector is used to measure this time difference. However, analog circuits and analog outputs are not practical in the digital world. Currently, in order to calibrate the voltage-controlled oscillator for reducing frequency errors is to measure the varactor and all other components in the voltage-controlled oscillator, compare these measurements to the circuit performance and calculate an expected range of within which the circuit is expected to perform.
Therefore it would be desirable to design a phase locked loop circuit that performs adjustment of the frequency of the voltage controlled oscillator by measuring and correcting itself in the field during operation. The present invention solves this problem in a unique and novel manner.
According to the present invention, a method for centering a phase detector within a phase locked loop circuit is provided. First a digital to analog converter is set to its center and a comparator is checked. If the comparator output voltage shows that an integrator output is higher than a reference voltage, then the output of the digital to analog converter is increased by one fourth its maximum. Similarly, if the inverse is true, the output is halved. The steps are repeated until the required precision is reached. The output of the integrator is then set to the reference voltage.